You pay gas spent * gas price amount of ether, where:

gas is a unit of computation

gas spent is the total amount of gas used in a transaction

gas price is how much ether you are willing to pay per gas

Transactions with higher gas price have higher priority to be included in a block.

Unspent gas will be refunded.

There are 2 upper bounds to the amount of gas you can spend

gas limit (max amount of gas you're willing to use for your transaction, set by you)

block gas limit (max amount of gas allowed in a block, set by the network)

In the below code we have a function with a infinite loop, it run until all of the gas are spent and transaction fails.